test(itest): Send status code during exceptional future completion

[jan-law]

Related #474

When a CompleteableFuture is completed exceptionally, assertRequestStatus() will also send the HTTP status code along with the exception. Integration tests can now verify that the correct error code was returned along with the exception message.

e.g

webClient
                .post("/api/v2/rules")
                .sendForm(
                        testRule,
                        ar -> {
                            assertRequestStatus(ar, response);
                        });

        ExecutionException ex =
                Assertions.assertThrows(ExecutionException.class, () -> response.get());
        MatcherAssert.assertThat(
                ((HttpStatusException) ex.getCause()).getStatusCode(), Matchers.equalTo(400));
        MatcherAssert.assertThat(ex.getCause().getMessage(), Matchers.equalTo("Bad Request"));

Edit: I have also added status code checks to RulesPostJsonIT.java and RulesPostFormIT.java.